Optical techniques for particle characterization are known. A laser based particle characterization technique is known. The optical techniques may be used to measure the size of particles, and may also be used to measure the concentration of these particles in a two-phase medium. The term xe2x80x98particlesxe2x80x99 may be used to refer to any second phase object within a first phase, and may include particles of matter, bubbles or droplets.
One of these techniques uses the so-called IMAX technique which has been described in xe2x80x9cNonintrusive Optical Single-Particle Counter For Measuring The Size And Velocity Of Droplets In A Sprayxe2x80x9d Applied Optics, Volume 23 No. 23, by Hess. In summary, the IMAX technique uses two laser beams. A smaller-diameter laser beam is used at the measurement location as a pointer beam. A larger diameter laser beam is used at the measurement location as a particle sizing beam. The size of a particle, which passes through the measurement location, may be inferred from the intensity of the scattered light from the particle sizing laser beams based on Mie scattering theory. This requires knowledge of the intensity of the laser at the location where the particle crosses the xe2x80x9cparticle sizingxe2x80x9d laser beam. The lasers have a Gaussian shape and therefore the intensity of the laser is a function of the trajectory of the particle relative to the particle sizing laser.
The pointer laser may decrease the uncertainty. Light which is scattered from the pointer laser is collected. This is used as evidence of the proper trajectory of the particle relative to the particle sizing laser. The pointer layer may also be used to verify that the particle is passing through the flat region of the particle sizing laser. This may reduce or eliminate uncertainty due to the Gaussian shape of the laser.
Particle concentration may be determined from the knowledge of the diameter of the pointer laser, and the velocity of the flow at the measurement location. However, there is still uncertainty in this technique.
Other techniques of determining particle concentration using optical techniques are known. These techniques may use the concept of Mie optical scattering to determine the concentration of particles.
The present application teaches detecting particle concentration using a special hollow shaped optical beam. In an embodiment, a detection may be made of whether the particle is passing through a specified location within the hollow shape. By detecting the particle beam passing only through this specified location, it becomes possible to even further reduce the uncertainty.